Diesel engines generate drive torque by drawing in and compressing air. Fuel is injected into the compressed air and the heat of compression induces auto-ignition of the air/fuel mixture. As a result, diesel engines do not include spark plugs to induce ignition of the air/fuel mixture. The air to fuel (A/F) ratio is regulated using open-loop control (i.e., no feedback). Combustion of the air/fuel mixture drives pistons within cylinders. In turn, the pistons drive a crankshaft that transfers drive torque to a drivetrain.
The torque output of a diesel engine is regulated based on a fueling rate and injection timing. The fueling rate and injection timing for a particular diesel engine is developed on an engine dynamometer. More specifically, dynamometer data is used to develop look-up tables for fueling rate and injection timing based on engine speed (RPM) and engine load. The look-up tables are programmed into the memory of the control module of each diesel engine.
Because the look-up tables are developed from dynamometer data for a particular diesel engine type, they are not calibrated or otherwise adjusted for each particular diesel engine. As a result, accuracy in the A/F ratio control is dependent on the extent that engine components and operation thereof (e.g., injector flow, mass air flow meter, engine volumetric efficiency) deviate from the diesel engine system used on the dynamometer.